The present invention relates to capacitive sensors, and more particularly, to the geometry of the capacitive sensors.
Touch-sensing devices, such as touch screens and touch pads, can include capacitive sensors that determine the location of an object proximate to the sensing device. The signals sensed by the capacitive sensors change with the object's presence and location relative to the sensors. For example, capacitive touch-sensing devices that employ a matrix of row and column electrodes as sensors can detect changes caused by the object in the capacitive coupling either between row and column electrodes, or between electrodes and a virtual ground.
In all capacitive touch-sensing devices, it is desirable to achieve the maximum resolution possible with the minimum number of row and column electrodes. Designs which achieve the minimum number of electrodes allow reduced amounts of sensing electronics or allow the same amount of sensing electronics to be used to create a larger sensing area.
Some touch-sensing devices utilize a simple array of sensors formed from rows and columns of straight electrodes with relatively uniform width. A problem with this configuration is that when an object that is small relative to the electrode spacing moves across the touch sensor, the detected signal on specific electrodes will have abrupt changes as the object moves from one electrode line to the next. For example, the object may begin entirely over on a first electrode line, and then next move into the space between this first electrode and a second electrode, and then move to entirely over the second electrode line. The abrupt signal changes that will occur are not optimal and may yield in uneven or rough pointing behavior.
Prior-art electrode geometries include traces of interconnected diamond patterns used in capacitive touch-sensing devices, as disclosed in U.S. Pat. No. 4,550,221. An alternate design has also been described in U.S. Pat. No. 6,147,680. However, the prior art designs still suffer from cross-axis performance problems. These problems are particularly encountered when the number of electrodes used to cover a given area of touch surface becomes too low. For example, if diamond patterns are used, as the number of electrodes used to cover a given area is decreased, the size of each diamond must be increased. If the size of individual diamonds in the pattern begins to approach the size of the object or finger to be sensed, loss of signal in one of the two sensor axes can lead to unsuitable loss of smooth pointing behavior. The same problem can occur with the more complex pattern disclosed in U.S. Pat. No. 6,147,680. An additional disadvantage of the pattern disclosed in the '680 patent is that it requires multiple crossings between each electrode aligned along one direction and each electrode aligned along the orthogonal direction. These multiple crossings can easily result in undesirably high levels of capacitive coupling between the sensor electrodes.
Another prior-art sensor pattern design is shown in FIG. 1. To differentiate between different electrodes in the drawing figures herein, distinct electrodes may be represented as dashed or solid lines of differing widths for convenience of viewing, this pattern is used on the Zytouch sensor available from Zytronic PLC, of Tyne & Wear, England. Like the pattern of the sensor in the '680 patent, this pattern employs multiple crossings that can easily result in undesirably high levels of capacitive coupling between the sensor electrodes.